Some drugs causes also a physical addiction. It's also about addiction for stimulation of pleasure zones of the brain. There's a complex of factors that plays role. But surely emotional attachment is also in charge. I know the cases when junkies stopped using drugs just because of a bad trips or a bad physical sensations. And there is also an interesting fact that junkies that addicted to injective type of drugs has also an addiction of injecting the needles in their veins.